Regeneration of blix solutions used in photographic processing

ABSTRACT

Photographic bleach-fix solutions employing a ferric salt of an aminopolycarboxylic acid as bleaching agent and a thiosulfate as fixing agent are regenerated by reducing the concentration of silver ion in the solution to a low level and mixing the solution with oxygen in an amount sufficient to convert substantially all ferrous ion in the solution to ferric ion. Removal of silver from the solution can be effected by addition of a chemical precipitant, by metallic replacement, or by electrolytic recovery and the necessary contact with oxygen after silver removal can be accomplished by aerating the solution. Treatment of spent bleachfix solutions in this manner will essentially restore their original bleaching and fixing capabilities and thereby permit their reuse in photographic processing.

United States Patent 72] Inventor Austin C. Cooley Rochester, N.Y. [21] Appl. No. 8,026 [22] Filed Feb. 2, 1970 [45] Patented Jan. 11, 1972 [73] Assignee Eastman Kodak Company Rochester, N.Y.

[54] REGENERATION 0F BLIX SOLUTIONS USED IN PHOTOGRAPHIC PROCESSING 8 Claims, No Drawings [52] US. Cl 96/60 BF, 96/60, 96/50 [51] Int. Cl G03c 5/32, G03c 5/26 [50] Field of Search 96/60, 60 BF [5 6] References Cited FOREIGN PATENTS 580,359 7/1959 Canada 96/60 801,106 9/l958 GreatBritain ABSTRACT: Photographic bleach-fix solutions employing a ferric salt of an aminopolycarboxylic acid as bleaching agent and a thiosulfate as fixing agent are regenerated by reducing the concentration of silver ion in the solution to a low level and mixing the solution with oxygen in an amount sufficient to convert substantially all ferrous ion in the solution to ferric ion. Removal of silver from the solution can be effected by addition of a chemical precipitant, by metallic replacement, or by electrolytic recovery and the necessary contact with oxygen after silver removal can be accomplished by aerating the solution. Treatment of spent bleach-fix solutions in this manner will essentially restore their original bleaching and fixing capabilities and thereby permit their reuse in photographic processing.

REGENERATKON OF BLIX SOLUTTONS USED IN PHOTOGRAPHIC PROCESSING This invention relates in general to photographic processing and in particular to the use of bleach-fix solutions. More specifically, this invention relates to treatment of spent bleach-fix solutions to substantially restore their original bleaching and fixing capabilities so as to permit their reuse.

Combined bleaches and fixes, commonly referred to as blixes, have been known for many years. They are used in processing silver halide photographic materials to simultaneously accomplish the steps of bleaching and fixing and thereby eliminate one step in the conventional processing procedure. The essential components of a blix solution are the bleaching agent, i.e. an agent which oxidizes the metallic silver in the silver image to a soluble form, and the fixing agent, i.e., an agent which dissolves the underdeveloped silver halide and the silver salts formed by the action of the bleaching agent. While a large number of different blix formulations are known to the art, of particular importance because of their stability and the excellent results obtained therewith are blixes in which the bleaching agent is a ferric salt of an aminopolycarboxylic acid and the fixing agent is a thiosulfate.

Blixes comprising a thiosulfate and a ferric salt of an aminopolycarboxylic acid are relatively costly and their commercial use has been hampered by this fact, especially in view of the further fact that heretofore they could be used only once and then had to be discarded. Thus, while it is known to regenerate bleach solutions and fixing solutions so as to permit their reuse, the methods employed are not useful for treatment of the aforesaid blix solutions. For example, ferricyanide bleach solutions can be regenerated by treatment with bromine, as is described in US. Pat. No. 2,515,930, or by treatment with reagents capable of releasing bromine in aqueous solution, as are described in US. Pat. Nos. 2,61 1,699 and 2,61 1,700, or by treatment with a water-soluble persulfate, as is described in US. Pat. No. 2,944,895. Reuse of spent fixing solutions is also rendered feasible by removing all or most of the silver from the fix and making the necessary chemical additions to reconstitute the original formulation. Methods of accomplishing this have been known for many years as evidenced, for example, by US. Pat. Nos. 1,446,405, 1,527,942 and 3,082,079. However, treatment of a blix formulation of the type described herein with bromine or a persulfate is not feasible as these agents will not only bring about the desired oxidation of ferrous ion in the solution but at the same time will destroy the ability of the solution to act as a fix by oxidizing thiosulfate ion to sulfate ion or other oxidation products. Accordingly, in the past, whenever these blix solutions have been employed they have been discarded after a single use, with resulting substantial economic losses as well as the creation of serious pollution problems. This has limited the use of these blixes to special situations, where the advantage of eliminating one processing step has outweighed these disadvantages, and essentially precluded their use in large scale continuous photographic processing.

In accordance with this invention, blix solutions in which a thiosulfate serves as the fixing agent and a ferric salt of an aminopolycarboxylic acid as the bleaching agent, are regenerated by a method which essentially restores their original bleaching and fixing capabilities and thereby permits their reuse. The method comprises reduction of the concentration of silver in the solution to a low level, as hereinafter described, and mixing of oxygen with the solution in a sufficient amount, as hereinafter described, to convert substan tially all ferrous ion in the solution to ferric ion. Appropriate additions of minor amounts of chemicals to restore the solution to its exact original composition are also made, as needed. By this means, the desired oxidation of ferrous ion to ferric ion is effected without significant destruction of thiosulfate so that the solution can be reused, with resulting elimination of the pollution problem and substantial savings in processing costs.

Blix solutions to which the method of this invention is applicable are solutions comprising a water-soluble thiosulfate which serves as the fixing agent. As is well known, thiosulfates which are useful for this purpose include ammonium thiosulfate and alkali metal thiosulfates such as sodium thiosulfate and potassium thiosulfate.

Use of ferric salts of aminopolycarboxylic acids as bleaching agents in blix formulations is also well known. Illustrative examples of the aminopolycarboxylic acids are the following:

nitrilotriacetic acid,

ethylenediamine tetraacetic acid,

diethylenetriamine pentaacetic acid,

ortho-diamine cyclohexane tetraacetic acid,

ethylene glycol bis(aminoethyl ether) tetraacetic acid,

diaminopropanol tetraacetic acid,

N-( 2-hydroxyethyl)ethylenediamine triacetic acid,

ethyliminodipropionic acid, and the like. The ferric salts of aminopolycarboxylic acids utilized in the practice of this invention may be salts in which all cations are the ferric ion or salts in which one or more of the carboxyl groups have formed a salt with a cation other than iron, e.g. with ammonia or with an alkali metal ion. An example of such a salt is sodium ferric ethylenediamine tetraacetate. The blix may also contain a nonchelated salt of an aminopolycarboxylic acid, e.g. the tetra sodium salt of ethylenediamine tetraacetic acid, in addition to the ferric salt.

While the above-described fixing agents and bleaching agents are the essential components of blix formulations to which the method of this invention is applicable, the blixes will usually also include other addenda known to the art to be useful for incorporation in blix formulations. Thus, for example, they may include an additional silver halide solvent such as a water-soluble thiocyanate, e.g. ammonium thiocyanate, sodium thiocyanate or potassium thiocyanate, as well as such compounds as ammonium bromide, alkali metal bromides, amines, sulfites, mercaptotriazoles, etc. The concentration of thiosulfate in the blix solution is typically from about 5 to about 200 grams per liter and the concentration of the ferric salt of an aminopolycarboxylic acid is typically from about 5 to about 200 per grams per liter.

Any of the methods which are known to the art for recovering silver from a fixing solution can be utilized in accordance with this invention to reduce the concentration of silver ion in the blix to the desired level. in general, these methods are of three types, namely, precipitation of the silver by addition of a chemical agent which forms an insoluble silver salt, metallic replacement in which the solution is brought into contact with a metal which is more electropositive than silver and thereby serves to replace the dissolved silver, and electrolytic recovery in which the silver is plated out on the cathode of an electrolytic cell. A discussion of the relative advantages and disadvantages of these methods and an extensive listing of the pertinent technical literature is included in an article by M. L. Schreiber entitled Present Status of Silver Recovery in M0- tion-Picture Laboratories", Journal of the SMPTE, Vol. 74, Pages 505 to 513, June, 1965.

For the purposes of this invention, it is preferred to recover the silver from the blix solution by metallic replacement, or by the electrolytic method, or by precipitation of silver sulfide through the addition of sulfide ion such as by adding sodium sulfide to the blix. Metallic replacement is conveniently and inexpensively accomplished by utilizing steel wool as the metal and employing the apparatus described in US. Pat. No. 3,369,801. The silver is effectively recovered from the blix by this procedure in spite of the fact that it is much more soluble in the blix solution than in a thiosulfate fix. When the steel wool cartridge is functioning effectively to remove silver from the blix solution, the color of the solution changes from deep red to light green, whereas this color change ceases to occur when the cartridge becomes exhausted. Thus, the color of the effluent from the cartridge provides a convenient indicator of its effectiveness and can be used to determine when a new cartridge should be employed. Electrolytic recovery should be carried out using a cell designed to operate with high agitation and high current density. The minimum current density needed to remove silver from the blix solution is higher than is required with a fixing solution while the tendency to formation of silver sulfide is less with the blix solution than with most fixing solutions so that use of high current densities is feasible. Good results in electrolytic recovery of silver from the blix solution will ordinarily be obtained with a current density of greater than about 7 amperes per square foot. Silver recovery cells which are particularly advantageous for use with the blix solution because they provide high agitation and permit the use of high current density are described in British Pat. Nos. 9l6,348 and 1,123,168.

To accomplish the objectives of this invention, the concentration of silver in the blix solution should be reduced to a level of less than about one gram of silver per liter of solution and more preferably to a level of less than about 0.5 gram of silver per liter of solution. The spent blix solution subjected to regeneration may have a silver concentration of up to about 10 grams per liter, or more, but will most usually have a concentration of silver of about 2 to 3 grams per liter.

The second step of the process of this invention is the step of mixing the blix solution, from which the silver has been removed by one of the methods discussed above, with oxygen in order to oxidize ferrous ion to ferric ion. Whereas the agents which have been used heretofore as oxidants in effecting regeneration of ferricyanide bleaches will cause destruction of thiosulfate, if used with the blixes described herein, it has unexpectedly been found that oxygen is effective in converting ferrous ion to ferric ion without destruction of thiosulfate. In treating the blix solution, oxygen should be employed in at least the stoichiometric amount, i.e. at least 0.25 mole of oxygen per mole of ferrous ion in the solution. Preferably, an amount of oxygen of at least about one mole per mole of ferrous ion is used. Any combination of oxygen flow rate and time which will permit adequate contact of the oxygen with the solution can be utilized. Of course, the amount of oxygen necessary will depend upon the composition of the blix being treated and the optimum amount of oxygen for a particular blix solution will depend upon the efficiency of the contacting procedure employed. Optimum amounts of oxygen in any particular instance may be readily determined by a few routine experiments.

Mixing of oxygen with the blix solution can be carried out using batch, semicontinuous or continuous techniques, as desired. In carrying out continuous treatment, a portion of the blix solution can be continuously withdrawn from the processing machine and returned as a recycle stream after regeneration. While pure oxygen can be employed to effect regeneration by the method of this invention, its use is not necessary and it will ordinarily be satisfactory to effect the oxidation by contacting the solution with air. In addition to the use of pure oxygen, or air, oxidation could be carried out using any otherwise inert oxygen-containing gaseous mixture, e.g. a mixture of oxygen and nitrogen or a mixture of oxygen and argon. Aeration will ordinarily be the simplest, most convenient and most inexpensive procedure. The time required for aeration will depend upon such factors as the characteristics of the air distributor, the flow rate, the size and shape of the tank in which the blix solution is aerated, the method used for removing the silver and so forth. Typical times required for substantially complete oxidation of ferrous ion by aeration are from about to about 90 minutes.

In practicing the method of this invention, the silver should, under ordinary circumstances, be removed from the solution before contact of the solution with oxygen is effected since the common procedures for silver recovery, e.g. metallic replacement or electrolytic recovery, will at the same time as they effect removal of silver bring about reduction of ferric ion to ferrous ion and thus counteract the effect of the oxidation step.

Following removal of the silver and aeration, or other mixing of the blix solution with oxygen, it is desirable to add small amounts of any necessary chemicals to establish the original concentrations and pH. The additions necessary will depend upon the method of silver recovery which was utilized and the extent to which the solution was contacted with oxygen. To determine the additions that are necessary, the solution can be analyzed using well-known analytical techniques.

The invention is further illustrated by the following examples of its practice.

Example 1 The blix employed in this example had the following composition:

(l Sodium ferric ethylenediaminetetraacetate (2) Tetra sodium salt of ethylenediaminctctraacetic acid This blix was used as replenisher in processing KODAK EK- TACOLOR paper in a KODAK Model 4c-3 paper processor. The seasoned blix overflow from the machine was passed continuously at a rate of ten gallons per hour through steel wool packed cartridges of the type described in US. Pat. No. 3,369,801, with two cartridges being connected together in series. Chemical analysis of the effluent from the second cartridge showed that the steel wool had removed essentially all of the silver from the blix and reduced most of the iron from ferric to ferrous. A l-liter sample of this effluent was placed in a 1,000-ml. graduated cylinder and a sintered glass air sparger was placed in the bottom of the cylinder and air was bubbled through the blix for one hour. Chemical analysis of the aerated blix showed that the aeration had oxidized substantially all of the ferrous iron in the desilvered blix back to the ferric form, thereby restoring the oxidation potential of the blix. Analysis of the seasoned blix, i.e. before regeneration, of the desilvered blix, i.e. after passage through the cartridges, and of the regenerated blix, i.e. after aeration, gave the following results:

The blix employed in this example had the following composition:

This blix was used in the same manner as described in example l and the seasoned blix overflow from the machine was passed through a single steel wool packed cartridge at a rate of 5 gallons per hour. The effluent from the cartridge was aerated for 1 hour using a -micron pore sintered stainless steel air sparger and an air flow rate of 40 cubic feet per hour. The aerated blix was then diluted by percent with water to aerated blix to be used as replenisher, the following chemical additions were made:

reduce the iron concentration and the following chemical ad- NaFe EDTA 6.2 g./l. ditions were made to the blix so that it could be reused as 5 $153 3s" I :1 g. replenish (N nispnroo'a soln.) 28 1111.11.

Nl-LSCN 1 g./l. Na,EDTA 53 71. The pH of the blix was adjusted to 7.0 and it was used as replenisher in the process. The regenerated blix was found to 10 b 1 1' m1 1 11 r 11 bl d h hoo x soln.) 60 e on y s 1g y ess act1ve t an res 1x an no p otograp 1c NH,SCN s gJl. d1fferences were observed as a result of its use. Analysis of The pH of the blix was adjusted to 7.0 and it was used as seasoned blix from the process using fresh replenisher, of the replenisher in the process. The regenerated blix was found to effluents from the electrolytic cell and from the aeration tank, be only slightly less active than fresh blix and no photographic and of seasoned blix from the process using regenerated differences were observed as a result of its use. 7 r H replenisher gave the following results:

Seasoned Bllx eflluent 1rom Seasoned blix from blix from fresh Electro- Aeration regenerated replenlsher lytlc cell tank replenlsher 7. 05 7. 05 7. 87 6. 98 1313:: 1. 84 0. 34 1 1 3; 3 NH4CNS, g./l 14. 7 17. 0 (NH4)2S2O:1 (60% 50111.), 1111/1 126 122 123 128 oz 3, g. 11. 6 11. 1 1. 05 11. 6 Total ED'IA (as NaiEDTA). g./l 69. 1 69 66. 7 69 Free EDTA (88 NaiEDTA), g./1 3. 21 3. 21 3. 21 3. 78 Total lron, g./l 9. 77 9. 75 9. 41 9. 62 Fe++, g./l 1. 20 5. 05 1. 33 0. 91 Fe g I] 8. 57 4. 70 8. 08 8. 71

Analysis of seasoned blix from the process using fresh replenisher, of the regenerated blix, i.e. after aeration, and of seasoned blix from the process using regenerated replenisher gave the following results:

Seasoned Blix Seasoned Blix from from Fresh Regenerated egeneralcd Replenisher Blix Replenisher pH 6.92 8.13 7.19 specific gravity l.l22 1.127 1.132 Ag 2.42 g./l. 0.01 g./l. 2.20 g./l. mucus 11.9 g./l. 14.8 g.ll. 13.4 ./1. (NH-02520.! (60% soln.) ll7 mL/l. llB ml./l. ll8 mL/l. Na,SO 7.2 gJl. 8.6 g./l. 9.4 g./l. Total EDTA (as Na,EDTA) 91.5 g./|. 97.6 g./l. 93.3 grll. Free EDTA (as Na EDTA) 40.4 g.l|. 9.3 gJl. 44.5 g./l. Total iron 7.53 g./l. 13.2 g./l. 7.20 g./l. Fe" 1.09 g./l. 0.4 g./l. 0.88 g.ll. F 6.44 g /l. 12.8 gJl. 6.32 3..

Example 3 an air flow rate of 15 cubic feet per hour. To enable the The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. A method of regenerating a used photographic bleach-fix solution to restore its bleaching and fixing capabilities and thereby permit its reuse in photographic processing, the bleaching agent being a ferric salt of an aminopolycarboxylic acid and the fixing agent a thiosulfate and said solution containing silver ions as a result of its use in photographic processing and ferrous ions formed by reduction of said ferric salt, which method comprises the steps of: (a) first removing said silver ions from said solution by chemical precipitation, metallic replacement or electrolytic recovery; and (b) second mixing oxygen with said solution in an amount sufficient to convert said ferrous ions to ferric ions, whereby the oxidation potential of said solution is restored without destruction of said thiosulfate so as to permit reuse of said solution in photographic processing.

2. The method as described in claim 1 wherein the concentration of silver is reduced to less than about 0.5 gram per liter before oxygen is mixed with the solution and oxygen is employed in an amount of at least about l mole per mole of ferrous ion.

3. The method as described in claim 1 wherein the oxygen is provided by aerating the solution.

4. The method as described in claim 1 wherein silver is removed from said solution by electrolytic recovery.

5. The method as described in claim 1 wherein silver is removed from said solution by metallic replacement.

6. The method as described in claim 1 wherein silver is removed from said solution by precipitation as silver sulfide.

7. The method as described in claim 1 wherein said thiosulfate is sodium thiosulfate.

8. The method as described in claim 1 wherein said ferric salt of an aminopolycarboxylic acid is a ferric salt of ethylenediaminetetraacetic acid. 

2. The method as described in claim 1 wherein the concentration of silver is reduced to less than about 0.5 gram per liter before oxygen is mixed with the solution and oxygen Is employed in an amount of at least about 1 mole per mole of ferrous ion.
 3. The method as described in claim 1 wherein the oxygen is provided by aerating the solution.
 4. The method as described in claim 1 wherein silver is removed from said solution by electrolytic recovery.
 5. The method as described in claim 1 wherein silver is removed from said solution by metallic replacement.
 6. The method as described in claim 1 wherein silver is removed from said solution by precipitation as silver sulfide.
 7. The method as described in claim 1 wherein said thiosulfate is sodium thiosulfate.
 8. The method as described in claim 1 wherein said ferric salt of an aminopolycarboxylic acid is a ferric salt of ethylenediaminetetraacetic acid. 